Sometimes the copyright of a text or image or other work expires and is not renewed. In this case it becomes known as Public Domain, and can be used by anyone without attribution of the original source. Also people may produce material specifically for the public domain. Such materials can be used by anyone, for any purpose, without restriction.
I am not sure where clipart fits here. I assume that many icons widely disseminated on the web (such as the www icon shown above) are likewise public domain.